1. Field of the Invention
The present invention relates to an electrode for use in solid-oxide fuel cells (referred to hereinafter as “SOFC”), sensors, solid state devices, and the like, in which the electrode is coated with ion conductive ceramic ceria film, extending a triple phase boundary where electrode/electrolyte/gas are in contact, thereby enhancing performance thereof, and a method for manufacturing the same. More particularly, the invention relates to a novel electrode in which a prefabricated electrode for use in a SOFC or sensor is coated with porous oxygen ion conductive ceramic ceria film by a sol-gel method, enabling an electron conductive path and an ion conductive path to be independently and continuously maintained, thereby solving an interconnection cut-off problem, enhancing electrode performance due to a great enlargement of triple phase boundary where electrode/electrolyte/gas are in contact, and further enabling the manufacture of the electrode at a lower temperature by employing a sol-gel method, resulting in preventing generation of undesired interfacial reaction products, and a method for manufacturing the same.
2. Description of the Related Art
In fabricating SOFCs, an electron conductive ceramic material, La1−xSrxMnO3 (referred to hereinafter as LSM) has been used as a cathode material (M. J. L. Ostergard and M. Mogensen, Electrochemica Acta. 38: 2015–2020, 1993; H. Kamata, A. Hosaka, Yuji Ikegami and J. Mizusaki, H. Tagawa, in first European Solid Oxide Fuel Cell Forum, eds. Ulf Bossel, Proceedings Vol. 2: 725–733, 1994). In a common method for manufacturing cathodes, an electron conductive material such as LSM, and an ion conductive material such as yttria-stabilized zirconia (referred to hereinafter as YSZ), that is, the electrolyte material, are mixed, considering a thermal expansion coefficient with an electrolyte, and the mixture is used to manufacture cathodes with high performance. Such cathodes are deposited on a dense electrolyte surface, which is made of an ion conductive solid oxide such as YSZ, fabricating a SOFC. Since the state-of-the-art SOFC now developed for a high capacity requires a high temperature of around 1000° C. for cell operation, there is a problem in that it is hard to find an interconnect material which is easy to process and is low in cost. In addition, such an SOFC has a disadvantage in that electrode particles are sintered upon long-term cell operation, decreasing an active area for reaction, thereby degrading cell performance (J. Mizusaki, H. Tagawa, K. Tsuneyoshi, A. Sawata, M. Katou, and K. Hirano, “The La0.6Ca0.4MnO3 YSZ composite as an SOFC air electrode”, Denki Kangaku, 58: 520–527, 1990). Accordingly, it is desired to lower temperatures for cell operation, and so many researchers have attempted to develop a low-temperature SOFC which operates at 500 to 800° C. Research to realize low operating temperatures in sensors and solid state devices is also actively underway. However, for general SOFCs, sensors and solid state devices which employ YSZ as the electrolyte and operate at 700 to 800° C., the cathode performance is so low, compared to anode performance, that it is necessary to improve the cathode performance.
Cathode performance of SOFCs is determined mainly depending on interfacial resistance, which is caused by generation of an interfacial reaction product such as SrZrO3 or La2Zr2O7 at a contact surface of the cathode with the YSZ electrolyte and on electrode polarization resistance, caused by electrochemical reactions occurring at the cathode. For this reason, improvement of cathode performance can be achieved by reducing those resistance values. Specially, as for a low-temperature SOFC which operates at 700 to 800° C., SOFC performance is considerably affected by electrode polarization resistance (Rel) according to a process of oxygen reduction at the cathode, and by interfacial resistance (iRinterface) between electrode and electrolyte (T. Tsai and S. A. Barnett, in Solid Oxide Fuel Cells V, eds. U. Stimming, S. C. Singhal, H. Tagawa and W Lehnert, The Electrochemical Society Proceedings Series PV 97–18, 368–375, 1997; M. Suzuki, H. Sasaki, S. Otoshi, A. Kajimura, N. Sugiura, and M. Ippommatsu, J. Electrochem. Soc. 141: 1928–1931, 1994). Thus, it is necessary for electrode polarization resistance at the cathode and interfacial resistance between the cathode and the electrolyte to be reduced, in order to manufacture a high performance SOFC.
A reaction (½O2+2e−→O2−) occurring at the cathode of a SOFC takes place mainly at the triple phase boundary in which the cathode, electrolyte and oxygen are in contact. The triple phase boundary is considerably affected by characteristics of a reaction occurring at a contact surface of the electrolyte and cathode. Therefore, it is desirable to form such a cathode/electrolyte interface where oxygen is diffused well and a contact area for the electrolyte and cathode is increased. As a result, resistance of the cathode/electrolyte interface is decreased, and also the area of triple phase boundary is increased, decreasing the electrode polarization resistance, thereby improving overall cathode performance. Methods for controlling microstructure of such a cathode to increase electrode performance include the following. One example is a two-dimensional method by which electrode powder with fine particles is well dispersed, thereby maximizing packing density in the electrode/electrolyte interface (M. Suzuki, H. Sasaki, S. Otoshi, A. Kajimura, N. Sugiura, and M. Ippommatsu, “High performance solid oxide fuel cell cathode fabricated by electrochemical vapor deposition”, J. Electrochem. Soc., 141(7): 1928–1931, 1994). Another example is a three-dimensional method by which electrode reactions occur even at portions of the electrode distal from the electrolyte, as well as at the triple phase boundary (T. Kenjo and M. Nishiya, LaMnO3 air cathodes containing ZrO2 electrolyte for high temperature solid oxide fuel cells, Solid State Ionics, 57: 295–302, 1992). Although such a three-dimensional method is desirable for improvement of electrode performance, it is applicable only to a mixed conductor, or a composite conductor having both ion- and electron-conductive paths. Accordingly, widely used are methods for manufacturing electrodes including the step of mixing electrode powder (electron conductive material) and electrolyte powder (ion conductive material), forming interpenetrating microstructures, thereby increasing electrode performance (U.S. Pat. No. 5,543,239). Recent reports disclose methods of infiltrating electrocatalysts into those electrodes for increasing cell performance (U.S. Pat. No. 6,017,647).
The simplest method of increasing a contact area of the electrode/electrolyte interface is to control temperatures in assembling the electrode and the electrolyte. In the course of fabricating a general SOFC which employs YSZ as the electrolyte, the cathode material coats the surface of the electrolyte, followed by sintering at 1100 to 1400° C., thereby assembling the cathode and the electrolyte. However, if the cathode material containing LSM is assembled with the electrolyte at temperatures above 1200° C., an interfacial reaction product such as SrZrO3 or La2Zr2O7 is generated at the electrode/electrolyte interface, resulting in degradation of SOFC performance (M. Mogensen and Steen Skaarup, Solid State Ionics 86–88, 1151–1160, 1996). On the other hand, if the sintering temperature is lower than 1200° C., the assembling of the cathode and the electrolyte is not achieved in an easy manner, so the interfacial resistance between the cathode and electrolyte becomes severe, resulting in degradation of SOFC performance.
As mentioned above, there are now commonly used technologies for increasing the cathode/electrolyte contact area and the area of triple phase boundary, comprising the step of solid state mixing LSM powder and YSZ powder, and using the mixture to manufacture cathodes having inter-penetrating microstructures (T. Kenjo and M. Nishiya, Solid State Ionics 57: 295–302, 1992; U.S. Pat. Nos. 5,543,239; 6,017,647). However, also in these cases, the temperature in assembling the electrode/electrolyte should be well controlled to fabricate high performance cells. That is, when assembling, an interfacial reaction product such as SrZrO3 or La2Zr2O7 is generated at the cathode/electrolyte interface, which degrades cell performance. In addition, where ion conductive powder and electron conductive powder are mixed, with an increase of the YSZ content, the electron conductive path may be broken, greatly increasing resistance, thereby causing an interconnection cut-off problem (D. W. Dees, T. D. Claar, T. E. Easler, D. C. Fee, and F. C. Mrazek, J. Electrochem. Soc. 134: 2141, 1987). Accordingly, where powder serving as an electron conductor is mixed with electrolyte powder to form an inter-penetrating microstructure, a minimum of a specific weight ratio or volume ratio of the electrolyte to the electrode must be employed to fabricate cells (U.S. Pat. Nos. 5,937,246; 5,993,988; and 6,017,647).
Virkar et al. proposed a new microstructure of an electrode where an electrochemical reaction is extended in a three-dimensional manner (U.S. Pat. No. 5,543,239). That is, an electrolyte is coated with slurry containing an electrolyte material, followed by thermal treatment, forming a porous electrolyte layer on the dense electrolyte. An electrocatalyst material serving as an electrode is infiltrated into the porous electrolyte layer. In such a way, the triple phase boundary is extended in a three-dimensional manner. This method, however, requires a high sintering temperature of approximately 1450° C. to assemble the dense electrolyte layer with the porous electrolyte layer formed thereon. In addition, there is a disadvantage in that if an electrocatalyst material serving as an electron conductor (that is, LSM, Pt, LSCF, etc) fails to sufficiently coat the entire porous electrolyte layer, internal resistance (IR) is considerably increased. Another disadvantage is that a low melting point of the perovskite-type material such as LSM or LSCF causes the electrode surface area to decrease upon long-term operation, decreasing the area for electrochemical reaction, thereby degrading cell performance. On the other hand, the microstructure of the electrode according to the invention, is formed by coating a porous electrode with a porous oxygen ion conductive film. This is in contrast to the method of Virkar et al., whose microstructure of the electrode is formed by coating the porous electrolyte with a porous electrode material. Advantageously, in accordance with the invention, a microstructure of the electrode is formed at lower temperatures using a sol-gel process. Further, the electrode having such a microstructure of extended triple phase boundary is able to prevent a surface area from being decreased due to the electrode sintering at high temperatures, while which is likely to be observed upon using Virkar's method, since the electrode and electrolyte are coated with an electrolyte material, such as Sm-doped ceria (SDC), which is harder to sinter than material.
Wallin et al. (U.S. Pat. Nos. 5,937,264; 5,993,986; and 6,017,647) reported that after manufacturing a cathode using a conventional method by which an electron conductive material and ion conductive material are mixed, the electrocatalyst in a solution state is then infiltrated into the cathode, thereby being capable of increasing electrode performance. However, this method has problems in that an interfacial reaction product is generated between the electron conductive material and the electrolyte and interconnection cut-off is caused, since the electron conductive material and ion-conductive material are mixed, forming the so-called interpenetrating networks.
Application of YSZ sol to SOFCs is disclosed in Japanese Pat. Nos. 06283179 and 02038362, and U.S. Pat. No. 5,993,988. In particular, Japanese Pat. Laid-open Publication No. Heisei 6-283179 discloses that YSZ slurry coats the support of the cathode or the anode, followed by thermal treatment. Cracks or pinholes caused by such thermal treatment may be filled up using YSZ sol, thereby forming the dense YSZ electrolyte thin film. On the other hand, in the invention, porous SDC film is formed on the electrode and electrolyte. As illustrated in FIG. 1, triple phase boundary of electrode/electrolyte/gas is continuously extended even to regions of the porous electrodes distal from the electrolyte, thereby decreasing electrode polarization. Thus, this approach is different from the above reference.
Ohara et al. (U.S. Pat. No. 5,993,988) teach that a solution of nickel acetate tetrahydrate is mixed with YSZ sol, followed by spray pyrolysis, preparing a composite powder whose NiO particles are surrounded by YSZ particles. The composite powder prepared by Ohara et al., may be used to prevent Ni particles from being sintered after reduction, owing to well-dispersed YSZ particles. This method is different from the method of forming a microstructure of the electrode of the present invention, as illustrated in FIG. 1. According to Ohara et al.'s method, the composite powder is prepared only when the amount of Ni:YSZ is in a specific range of 90:10 to 50:50 mole %. This is based on the fact, as mentioned above, that with an increase of the YSZ content, electron conductivity is greatly decreased, causing an interconnection cut-off problem. Moreover, the above method is based on the fact that YSZ particles partially cover Ni particles so Ni particles are prevented from being sintered by a pinning effect, but in this case the electron conductive path is broken, so the YSZ particles fail to form a continuous triple phase boundary. For this reason, Ohara's method cannot maximize performance of the electrode, while the present invention achieves maximized electrode performance.